Self-Injurious Behavior (SIB) is a significant medical and educational problem in institutionalized, mentally retarded persons. The objective of this research is to determine if the endorphin-reinforcement model of SIB (Davidson, Kleene, Carroll, & Rockowitz, 1983; Deutsch, 1986; Sandman, Datta, Barron, Hoehler, Williams, & Swanson, 1983) can provide the basis for an effective medical treatment for these dangerous behaviors. This model suggests that SIB is maintained by the release of endogenous opioids when these behaviors occur. These endorphins may act as a reinforcer for the behavior and/or may blunt the natural punishment process due to their analgesic effects. The evaluation of this model will be accomplished by assessing the effects of the delivery of an opioid antagonist, Naltrexone Hydrochloride (Trexan, DuPont Pharmaceuticals), on institutionalized, mentally retarded adults presenting a variety of different self-injurious behaviors that have proven resistant to behavioral and other medical treatments. If the action of the endorphins are maintaining these behaviors, the delivery of this opioid antagonist should suppress its effects and produce extinction of SIB. If this reduction is SIB is not found, several alternative hypotheses about the influence of endorphins on this behavior will be tested. These include an assessment of the possible differential response to the medication across various categories of SIB and subject types, and, the effect of elevated doses and extended treatment durations with the medication.